Transition metal dichalcogenides (TMDs) are a class of semiconductors which can be easily exfoliated (i.e. cleaved) into atomically thin layers. At the monolayer limit, TMDs become direct-gap semiconductors and possess exciton resonances with large binding energies, on the order of hundreds of millielectronvolts. We fabricate van der Waals heterostructures in which monolayer TMDs are encapsulated between two hexagonal boron nitride flakes. We gate these passivated monolayers by adding an additional graphene monolayer using a dry transfer method. We study the photoluminescence and absorption of these high-quality samples at cryogenic temperatures (T=4K) as a function of the Fermi level, which we modulate via an applied gate voltage. We measure the linewidth of the neutral exciton to be less than 1nm, and find that our samples are spectrally homogeneous (variations within a linewidth) over several micrometers.